While conventional bypass graft treatment of aneurysms has steadily improved, mortality rates continue to be relatively high in cases such as abdominal aortic aneurysms. These often asymptomatic aneurysms 15 of blood vessel 16, as shown in FIG. 1, generally progressively enlarge in most patients over time, increasing the risk of rupture. Traditional bypass grafts are then required which are extremely invasive and include all the risks of open surgeries such as paraplegia, renal insufficiency, and myocardial infarction. Moreover, even three (3) to five (5) years after these surgeries, complications may arise which include concomitant coronary atherosclerotic disease, graft infection, aortoenteric fistula, thromboembolish, and anastomotic aneurysms.
In the recent past, more innovative approaches have evolved for the treatment of aneurysms. For example, DACRON.RTM. grafts, endovascular stent grafts and covered stents (referred heretofore generally as "stent grafts"), which have rapidly developed in an effort to expand stent technology, may be employed as a means of aneurysm treatment. These hybrid devices combine graft material with a stent or stent-like device to provide an expandable, stent-like structure having an impervious luminal surface.
These combination of features, once implanted, are very conducive to achieve endovascular exclusion of aneurysms. Typically, a graft material is mounted to and positioned along an exterior circumferential surface and/or the interior circumferential surface of the prosthesis in a manner forming an endovascular, blood impervious lumen therethrough. A proximal end of the graft is preferably endovascularly positioned just upstream from the vascular disorder while a distal end thereof terminates at a position just downstream thereof. As the proximal end and the distal end of the stent graft become anastomosed with the vessel wall, the vascular disorder becomes endovascularly excluded from the blood flow while the stent graft impervious lumen maintains vessel patency.
Upon proper endovascular deployment and seal formation of the stent, cell matrix formation and tissue healing may commence in the aneurysmal sac and on the luminal surface. For example, in the aneurysmal sac between stent graft and the vascular wall, the residual blood clotting and inflammatory response cause cellular proliferation and connective formation, forming a matrix that may seal the sac. In addition to the sealing, the resulted wall, which is a combination of prosthesis, connective tissue matrix, and arterial wall provides a conduit support of proper hemodynamic blood flow.
Intraluminally, thromboembolic processes will occur on the luminal surface of the graft/stent. Briefly, during this thrombotic phase, platelets and blood clots adhere to the surface to form a fibrin rich thrombus. Endothelial cells then appear, followed by intense cellular infiltration. Finally, during the proliferative phase, actin-positive cells colonize the residual thrombus, resorbing the thrombus.
The primary problem associated with this technique is the time period required for endovascular sealing and repair of the aneurysmal sac. Tissue response to injuries of this nature are generally on the order of a few months to years. This is especially true for the luminal surface of the graft material where organized thrombus formation may be difficult to achieve. Such endothelial cell growth to line the lumen of the stent graft may require years of healing or may never be fully completed.
Accordingly, several clinical complications may result due to improper delayed cellular healing. One of the most prevalent problems, aortoentenic fistula, arises when the seal integrity between the vessel wall and the proximal end of the stent graft is compromised due to slow thrombus formation and incomplete tissue growth. Such upstream, proximal seal breaches cause blood infiltration through the incomplete anastomosis that may lead to abdominal blood loss. Stent grafts efficiency and effectiveness are substantially reduced since the luminal surface is not re-endothelialized, exposing the foreign surface to the risk of thrombosis and its complications.
There is a need, therefore, to increase the effectiveness and efficiency of the stent graft to reduce the time period for vascular repair.